Irssi (Finnish pronunciation: [ˈirsːi] (listen)) is an Internet Relay Chat (IRC) client program for Linux, FreeBSD, macOS and Microsoft Windows.

[1] The program has a text-based user interface was written from scratch using C. It may be customized by editing its config files or by installing plugins and Perl scripts.

[6] This freed the developers from having to deal with the constraints of an existing codebase, allowing them to maintain tighter control over issues such as security and customization.

[5] Irssi was written primarily to run on Unix-like operating systems, and binaries and packages are available for Gentoo Linux, Debian, Slackware, SUSE (openSUSE), Frugalware, Fedora, FreeBSD, OpenBSD,[12] NetBSD, DragonFly BSD, Solaris,[13] Arch Linux,[14] Ubuntu,[13] NixOS,[15] and others.

[16] For the Unix-based macOS, text mode ports are available from the Homebrew, MacPorts, and Fink package managers, and two graphical clients have been written based on Irssi, IrssiX, and MacIrssi.